1. Field of the Invention
The present invention relates to a method of moulding and curing tires for vehicle wheels, comprising the following steps: disposing a tire being processed on a toroidal support the outer surface of which substantially mates an inner surface of the tire itself; closing the tire and the toroidal support inside a moulding cavity defined in a vulcanization mould, said moulding cavity having walls the shape of which matches that of an outer surface of the tire when vulcanization has been completed; pressing the tire by its outer surface against the moulding cavity walls; administering heat to the tire being processed to cause a molecular crosslinking of same.
The invention also relates to an apparatus for moulding and curing tires for vehicle wheels, comprising: a toroidal support arranged to engage a tire being processed, said toroidal support having an outer surface substantially mating an inner surface of the tire itself; a vulcanization mould arranged to receive the toroidal support carrying the tire being processed within a moulding cavity having a tire-holding space delimited between the outer surface of the toroidal support and walls of the moulding cavity itself mating an outer surface of the cured tire; pressing devices for pressing the outer surface of the tire against the inner wall of the mould; heating devices for transmitting heat to the tire enclosed in the moulding cavity.
2. Description of the Related Art
In a tire production cycle it is provided that, after a manufacturing process in which the different tire components are made and/or assembled, a moulding and curing process should be carried out for the purpose of stabilizing the tire structure to a given geometric conformation, generally characterized by a particular tread pattern.
To this aim, the tire is introduced into a vulcanization mould usually comprising a pair of cheeks adapted to be axially moved close to each other, which are arranged to operate on the tire bead and sidewalls, and at least one crown of circumferentially distributed sectors susceptible of being radially moved close to each other so as to operate at the tire tread band. In more detail, cheeks and sectors are mutually movable between an open condition, in which they are spaced apart from each other to enable loading, of the tires being processed, and a closed condition in which they define a moulding cavity the geometric conformation of which is the same as the outer surfaces of the tire to be obtained.
In one of the most widespread moulding methods it is provided that a vulcanization bladder of elastomer material filled with high-temperature and high-pressure steam and/or another fluid should be inflated at the inside of the tire enclosed in the moulding cavity. In this manner the tire is conveniently urged against the inner walls of the moulding cavity and stabilized to the geometric configuration imposed to it, following a molecular crosslinking to which the elastomer material of which it is made is subjected, due to heat transmitted by the fluid through the bladder and by the mould walls.
Also known are moulding methods in which, instead of an inflatable vulcanization bladder, a rigid toroidal support having the same configuration as the inner surface of the tire to be obtained is arranged within the tire.
Such a method is disclosed for example in the European Patent EP 242 840 in which a rigid toroidal support is employed for imposing an appropriate shape and definitive sizes to a tire enclosed in the mould. According to the above patent disclosure, the different coefficient of thermal expansion between the toroidal metal support and the raw elastomer material of which the tire is made is utilized for achieving an appropriate moulding pressure.
In conclusion, the assembly of the parts forming the mould and the toroidal support define a closed space in the moulding cavity which is exactly shaped like the whole geometric configuration of the tire. In this way, both the outer surfaces and the inner surfaces of the tire are maintained in contact with rigid portions of the moulding and curing apparatus. In other words, all parts of the apparatus that are intended for setting the final tire geometry are rigid parts, in contrast with the methods using an inflatable vulcanization bladder that, as known, constitutes a deformable portion of the mould.
It is the Applicant's perception that, at the present state of the art, both the methods using an inflatable vulcanization bladder and the methods using a rigid toroidal support during tire vulcanization have some problems.
With reference to the methods using an inflatable bladder, it is in fact to be noted that the bladder deformability can easily give rise to geometric and/or structural imperfections in the tire due to possible distortions suffered by the bladder itself, following an unbalanced expansion for example, and/or due to friction phenomena generated between the outer surfaces of the bladder and the inner surfaces of the green tire.
Since on the bladder itself also relies the task of locking the tire beads against the corresponding mould portions, the bladder deformability makes it difficult to reach sufficiently high pressures for bead locking. Thus, undesired misalignments of the beads relative to the geometric axis of the tire may occur, giving then rise to distortions of the whole tire structure. In addition, an insufficient pressure for bead locking may cause formation of flashes at the beads, due to leakage of the elastomer material between the bladder and the mould, above all at the starting instants of the vulcanization process.
The vulcanization bladder needs use of important amounts of steam since the whole inner volume of the bladder inflated in the mould cavity is to be filled up, and in addition it constitutes an obstacle to transmission of heat to the tire by steam itself.
On the other hand, use of a rigid toroidal support instead of the inflatable vulcanization bladder makes it necessary to carry out a very precise and difficult checking of the volumes of the material employed in manufacturing the tire.
In addition, it is presently impossible to impose an appropriate radial and/or circumferential expansion to the tire, for achieving desired preloading effects in the reinforcing structures employed in the tire manufacture, for example.
Furthermore, even with the aid of the rigid toroidal support, achievement of a correct and efficient heat transmission to the inside of the tire is rather difficult.
U.S. Pat. No. 1,798,210 discloses a curing method according to which a previously-manufactured green tire is fitted on a toroidal support made of vulcanized rubber, to be then closed in the moulding cavity defined in a vulcanization mould. The toroidal support is completely hollow and is such shaped and sized that it cooperates with the inner walls of the moulding cavity to carry out a hermetic seal at the inner circumferential edges of the tire. The toroidal support sizes however are smaller than the inner sizes of the green tire, so as to define a gap extending from one bead to the other between the outer surfaces of the toroidal support and the inner surfaces of the green tire. After carrying out the closure of the mould, hot water and/or another hot fluid under pressure is admitted to the toroidal support, which fluid reaches the above described gap through openings formed in the toroidal support for fulfilling all functions required for tire moulding and curing.
In this curing process, however, manufacturing of the tire directly on the toroidal support to be introduced into the vulcanisation mould together with the tire itself is neither provided nor allowed.
In addition, since the toroidal support necessarily has smaller sizes than the inner sizes of the tire, structural defects may easily arise, which result from an imperfect centering and/or uncontrolled movements or distortions to which the tire is submitted on its being closed in the moulding cavity.
The Applicant has perceived that important improvements may be achieved if admission of working fluid, for carrying out tire moulding and/or heat supply for vulcanization, takes place at the inside of a gap which is formed between the toroidal support and the green tire only following an expansion imposed to the tire by effect of pressure. A method and an apparatus conceived on the basis of this principle are the object of a European Patent Application No. 98830473.9 in the name of the same Applicant.